1. Field of the Invention
The present invention relates to an analog switch circuit for activating a number of analog switches made up of semiconductor switching elements.
2. Description of the Prior Art
Conventional analog switches can roughly be classified into two groups mechanical and semiconductor. The mechanical switches are reed relay switches, while the semiconductor switches are insulated gate field effect transistors (referred to as FETs simply, hereinafter). However, FETs are more suitable for use as an analog switch because no gate current flows through the gate thereof without producing a dc offset.
In the above-mentioned FET analog switch mode up of FETs, when a high speed switching signal is directly applied to the gate of an FET to turn it on, since current changes at high speed between the source and the drain thereof, there exists a problem in that impulse switching noise will be generated whenever the analog switch is turned on or off.
Therefore, where a analog switch circuit turned on or off in response to high-speed switching signals is incorporated in an audio appliance, there arises a problem in that generated switching noise exerts a harmful influence upon the audio appliance and therefore the quality of the audio appliance will be degraded. In addition, in case the generated switching noise level is extraordinarily high, there exists a danger such that a speaker provided for an audio appliance may be damaged.
To overcome the above-mentioned problem, the analog switch circuit is usually configured in such a way that switching signals are not directly applied to the gates of FETs.
FIG. 1 shows an example of prior-art analog switch circuits which can reduce switching noise. This prior-art analog switch circuit includes two analog switches 5 each made up of a pair of parallel-connected P-channel FET (referred to as PFET) and N-channel FET (referred to as NFET) with source and drain terminals of two FETs connected to each other. Further, an integrating circuit 7 composed of a resistor R and a capacitor C is connected to each gate terminal of both the PFET 1 and NFET 3, respectively.
Therefore, each analog switch 5 is turned on or off in response to two switching signals outputted from two integrating circuits 7 on the basis of a control signal applied through the control circuit 9. That is, the analog switches 5 are turned on or off in response to two switching signals having gentle leading and trailing edges, so that the PFET 1 and the NFET 3 are both turned on or off at relatively low switching speed, thus reducing change rate per unit time of drain currents passed through the PFET 1 and NFET 3 whenever these FETs 1 and 3 are turned on or off, in order to suppress the generation of switching noise.
In the above-mentioned prior art analog switch circuit, however, since an integrating circuit is connected to each gate terminal of each FET which configures an analog switch and further the resistance or the capacitance of the integrating circuit each occupies a relatively large area, where the analog switch circuit including integrating circuits is formed into an IC circuit, there exists a problem in that a relatively large area is necessary to form these resistances and capacitors. In particular, in the case of an electronic variable resistor circuit, for instance which requires a great number of analog switches, the above-mentioned problem is serious.
In addition, where a great number of integrating circuits are formed to reduce noise, since there inevitably exist differences in resistance and capacitance between the integrating circuits, and therefore differences in time constant therebetween, another problem will occur such that switching noise generation cannot be prevented perfectly.